I don't think you can unfortunately. When a new firmware is released they want it to be permanent since its meant to "improve" the console so I don't think you can downgrade at all.

 

JTAG ports are factory-disabled on Sony products and it's unlikely that they forgot to do so with the Vita, and seeing that the Kernel Mode of the Vita is not cracked yet, there's no way anyone will be able to downgrade their systems anytime soon. User Mode access which is currently being developed will not allow for system modification - it can be used for homebrew purposes only.